The present invention relates to a booster installed to a brake apparatus of a vehicle such as an automobile, and in particular, to an electric booster using an electric actuator as a boosting source.
As disclosed in, for example, Japanese Patent Public Disclosure Nos 2007-191133 and 2007-112426, there is known an electric booster including an input piston connected to a brake pedal, and an assist piston to be driven by an electric actuator (electric motor) which are inserted in a master cylinder, and configured to move the assist piston forward by controlling an operation of the electric motor by a controller based on a relative position between these pistons, thereby generating a desired hydraulic pressure in the master cylinder. This electric booster includes a neutral spring for biasing the relative position between the input piston and the assist piston to a predetermined neutral position, and thereby can carry out various kinds of brake control such as the boosting control, the brake assist control, and the regenerative cooperative control without causing an uncomfortable feeling against a pedal operation by adjusting a reaction force to the brake pedal with the aid of the spring force of the neutral spring which is generated according to the relative position between the input piston and the assist piston. Further, this electric booster can carry out the automatic brake control by moving the assist piston forward by causing the controller to drive the electric motor according to, for example, a running condition of the vehicle without relying on a brake pedal operation.
For example, in the vehicle follower control, which functions to maintain a predetermined distance to a preceding vehicle, a driver's pressing of the brake pedal during execution of the automatic brake control triggers a stop of the automatic brake control, and switches the brake control to such control that the driver's brake operation is prioritized (override is caused by a brake operation). In this case, the conventional techniques achieve a stop of the automatic brake control by monitoring a displacement of the brake pedal, and determining that a driver operates the brake pedal when the brake pedal is displaced.
However, as the advancing amount of the assist piston is increased during the execution of the automatic brake control, this may cause the input piston to be pulled in toward the master cylinder by the assist piston through the neutral spring, resulting in a displacement of the brake pedal. Therefore, only by monitoring a displacement of the brake pedal, it is difficult to accurately detect whether the brake pedal is displaced because the brake pedal is pulled in along with an advance of the assist piston during the automatic brake control, or because a driver operates the brake pedal.